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Nodes: Support internal links for custom sockets

Currently, nodes with custom sockets do not get their internal links
populated. So operators like Delete And Reconnect don't work with such
nodes. This patch put custom sockets into consideration when computing
priorities for internal links such that sockets of the same idname get
connected. Additionally, the patch cleanup the function in the process
to avoid redundant code repetition.

Reviewed By: Jacques Lucke

Differential Revision: https://developer.blender.org/D13386